Mounting global energy demands have dramatically increased the cost of fossil-fuel-based energy sources and petrochemicals. And, the environment has been harmed, perhaps irreparably, in an effort to meet this demand by discovery and extraction of fossil-fuel feedstocks, and by processing of those raw feedstocks to produce ever increasing amounts of fuel, petrochemicals, and the like. Petrochemicals furthermore provide the majority of raw materials used in many plastics and chemical industries. The present invention is directed to providing isolated, plant-derived, renewable and sustainable compositions that have multiple utilities and that provide renewable and sustainable substitutes for fossil-fuel derived and petrochemical feedstocks.
Lignin is a complex, high molecular weight polymer that occurs naturally in plant materials, and is one of the most abundant renewable raw materials available on earth. Lignin is present in all vascular plants and constitutes from about a quarter to a third of the dry mass of wood. It is covalently linked to hemicellulose in plant cell walls, thereby crosslinking a variety of plant polysaccharides. Lignin is characterized by relatively high strength, rigidity, impact strength and high resistance to ultra-violet light and, in wood, has a high degree of heterogeneity, lacking a defined primary structure.
Lignin molecules are generally large, cross-linked macromolecules and may have molecular masses in excess of 10,000 in their native form in plant material. The degree of lignin polymerization in nature is difficult to determine, since lignin is fragmented during extraction. Various types of lignin have been characterized and described, with the lignin properties generally depending on the extraction methodology. There are three monolignol monomers, which are methoxylated to various degrees: p-coumaryl alcohol, coniferyl alcohol, and synapyl alcohol. These monomers are incorporated in lignin polymers in the form of phenylpropanoids p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S). Different plants exhibit different proportions of the phenylpropanoids.
The polyphenolic nature of lignin and its low toxicity, together with many additional properties (such as its dispersing, binding, complexing and emulsifying, thermal stability, specific UV-absorbing, water repellent and conductivity characteristics), make it an attractive renewable replacement for toxic and expensive fossil fuel-derived polymer feedstocks. Unlike synthetic polymers, lignin is biodegradable in nature. In spite of its biodegradability, lignin is known to be one of the most durable biopolymers available.
Large quantities of lignin are produced as a by-product of the pulp and paper industry. Despite its unique and desirable characteristics as a natural product with multiple beneficial chemical, physical and biological properties, and its abundance, lignin isolated from plant materials remains largely under-exploited. The heterogeneity and low reactivity of lignin recovered from waste effluent produced by the pulp and paper industry has resulted in limited industrial utilization of this highly abundant and renewable natural product.
Lignin is recovered from sulfite or Kraft wood pulping processes as lignosulfonates containing significant amounts of contaminants. The recovered lignin molecules lack stereoregularity, with repeating units being heterogeneous and complex. In general, lignin obtained as a by-product of the Kraft process (referred to as Kraft lignin) requires further processing and/or modification, as described in U.S. Pat. Nos. 5,866,642 and 5,202,403, in order to increase its reactivity and to allow its use in the formation of higher value products. Kraft lignin preparations contain a mixture of lignin sulfonate and degraded lignin, together with numerous decomposition products, such as sugars, free sulfurous acid and sulfates. The phenolic structures of the Kraft lignin are highly modified and condensed. The sulfite process for wood treatment produces a water soluble sulfonated lignin preparation that contains a high content of sugars, sugar acids and sugar degradation products, as well as resinous extractives and organic constituents with multiple coordination sites. The costs associated with the purification and functionalization required to make these low grade lignin preparations useful have limited their utilization in high value application markets.
The use of organic solvents for lignin extraction prior to carbohydrate hydrolysis as disclosed, for example, in U.S. Pat. Nos. 4,764,596, 5,788,812 and 5,010,156, was shown to improve the quality of the resulting lignin, but the use of a catalyst in combination with various types of solvents under severe conditions often produced lignin having altered reactivity (McDonough (1992) TAPPI Solvent Pulping Seminar, Boston, Mass., The Institute of Paper Science and Technology; Pan and Sano (2000) Holzforschung 54:61-65; Oliet et al. (2001) J. Wood Chem. Technol. 21:81-95; Xu et al. (2006) Industrial Crops and Products 23:180-193).
The reactivity of lignin depends mainly on the presence and frequency of aliphatic, phenolic hydroxyl and carbonyl groups, which varies depending on the lignin source and the extraction process used to obtain the lignin. The average molecular weight and polydispersity of lignin in the preparation also has a great impact on its reactivity.
As demonstrated in the many attempts to replace phenol with lignin in the formation of phenol-based resins, the low reactivity of the lignin means that only a small amount of phenol can be displaced without affecting the mechanical and physical properties of the final resin (etin and Özmen (2002) Int. J. Adhesion and Adhesives 22:477-480; etin and Özmen (2003) Turk. J. Agric. For. 27:183-189; Sellers et al. (2004) For. Prod. J. 54:45-51). Similar difficulties are encountered when lignin is employed in other types of applications. For example, the thermostability of lignin used to produce carbon fibers by spinning, as described in U.S. Pat. No. 6,765,028, and the carbonization of the resulting lignin fibers, are largely influenced by the method of lignin extraction and the origin and composition of the lignin (Kadla et al. (2002) Carbon 40:2913-2920).
When acidic ethanol-extracted lignin was used as a polyol for the experimental preparation of polyurethane (PU), replacement of 35% to 50% of the PU resin was achieved without compromising the integrity of the lignin-based PU film (Vanderlaan and Thring (1998) Biomass and Bioenergy 14:525-531; Ni and Thring (2003) Int. J. Polymeric Materials 52:685-707). Smaller ratios of replacement of PU resin (<10%) have been achieved by direct blending of soda lignin in pre-formed PU resin (Ciobanu et al. (2004) Industrial Crops and Products 20:231-241).
Polymer blending is also a convenient method to develop lignin based products with desirable properties. (See, e.g., Kubo and Kadla (2003) Biomacromolecules 4(3):561-567; Feldman et al. (2003) J. Appl. Polym. Sci. 89:2000-2010; Alexy et al. (2004) J. Appl. Polym. Sci. 94:1855-1860; Banu et al. (2006) J. Appl. Polym. Sci. 101:2732-2748) The efficiency and quality of the polymer blend is normally closely related to the chemical and physical properties of the lignin preparation, such as monomer type(s), molecular weight and distribution, which depends on the origin of the lignin and method used for its extraction, isolation and harvesting.
Upgrading lignin through chemical functionalization has been shown to be a good strategy for the successful incorporation of plant-derived lignins in high value products. However, these reactions are costly when low grade or low reactivity lignin is used as the substrate for chemical modification. Large amounts of reactants are required, together with longer reaction times and higher temperatures, to achieve relatively low rates of transformation of low grade and low reactivity lignins. This adds to the cost of the lignin feedstock and reduces its desirability for use in various types of industrial processes.